The present invention relates to a heavy-duty pneumatic radial tire, and more specifically, a heavy-duty pneumatic radial tire having an improved durability.
Conventionally, in heavy-duty pneumatic radial tires (hereinafter referred to simply as heavy-duty tires), as shown in FIG. 2 of the accompanying drawings their bead portion 3 comprises a single carcass layer 1 having its respective end portions turned up about a bead core 2 from the inner side to the outer side of the tire in a manner of enrobing a rubber stock 5, and a single reinforcing layer 6 also turned up from the inner side to the outer side of the tire along a portion of the carcass layer 1. In the bead portion 3 of such heavy-duty tires, there exists a point of discontinuation of rigidity in the vicinity of the end edge portion indicated at 1.sub.E of the carcass layer 1. Thus, when the bead portion 3 undergoes a deformation, there tends to occur the generation of strain or the concentration of stress in the above point of discontinuation of rigidity, to produce a separation trouble, giving a rise to a lowering of the durability of the tire.
In order to prevent the generation of strain and suppress the stress concentration in the vicinity of the end edge portion 1.sub.E of the carcass layer 1, attempts have been made to increase the rigidity of a rubber layer in the bead portion which is contacted with a rim on which the tire is mounted or to cover the end edge portion 1.sub.E with a reinforcing layer 4 to thereby suppress the possibility of movement around the bead core 2 and suppress the deformation of the bead portion 3. However, a limitation is applicable to the possibility of increasing the rigidity of a rubber layer to be applied about the bead core 2 and also to the tenacity of the reinforcing layer 4, so that even according to the above attempts, it has been impossible to sufficiently suppress the movement around the bead core and the deformation of the bead portion.
Also, the above reinforcing layer 6 applied along a portion of the carcass layer 1 at a cord angle relative to the radial direction of the tire is turned up around the bead core 2, so that cords on the inner side of the bead portion cross cords on the outer side of the bead portion. Therefore, when the bead portion undergoes a deformation, the reinforcing layer 6 tends to undergo a twist, whereby the concentration of stresses applied to the end edge portion of the bead portion tends to be promoted and the generation of strain cannot be suppressed, resulting in a lowering of the durability of the bead portion.